Talk to any modern reefkeeper and very soon the subject of nitrate will arise. The bane of many a reefkeeper over the last thirty years, rising nitrate and ultimately uncontrollable nitrate levels have long been blamed for the browning of corals, and horrible, nuisance algae. But nitrate and its importance to the marine aquarium has now come full circle, and in three, recent back-to-back conversations with accomplished reefkeepers, all three complained that they didn’t have any nitrate, and were actively looking to create some! That’s the marine hobby...

What is nitrate

Nitrate is the bi-product of biological filtration. Toxic ammonia is produced by fresh and saltwater fish. Beneficial filter bacteria convert ammonia into nitrite, and then into nitrate, as part of the Nitrogen Cycle, and Nitrate would then be consumed by plants as fertiliser, diluted in vast water bodies, or physically removed by you, in water changes.

What does nitrate do

Nitrate isn’t much of a problem to marine fish, but it can be to corals, as they are adapted to very low levels of nitrate in the oceans. Nitrate is a natural fertiliser, readily utilised by plants and algae, and when exposed to high nitrate levels the zooxanthellae within the coral tissue blushes brown. It doesn’t necessarily damage the coral, it’s a nutrient, but at a basic level it will mean that your corals may have any desirable colours masked by the brown, and you’re pretty certain to develop undesirable growths of algae on the rocks, which can overgrow and smother corals.

Nitrate in natural reefs

On the natural reef, elevated levels of nitrate, or nitrogen fertiliser run-off from farms, is often blamed for their decline. Just like in an aquarium, nitrates will cause excessive algae growth and smother corals, sending the reef out of balance. All nutrients should be tightly cycled on a coral reef, preventing nuisance algae, so excessive algae is a sign of pollution, elevated nutrient levels, and an indicator of an unhealthy reef.

Sources of nitrate in the home aquarium

In the home marine aquarium nitrate is caused mainly by fish, and the ammonia they excrete. It can be caused by the breakdown of any organic material by nitrifying bacteria, so that would also include uneaten food, dead fish or corals, and can leach from natural decoration like rocks and sand. 

Nitrate is present in tap water, so needs to be removed by reverse osmosis or deionisation before its used for topping up or mixing with salt, and a bad salt could potentially include nitrate, although most are labelled as nitrate and phosphate free. Soak new decor in RO water and test before and after for nitrate, just to be sure. “Dead” live rock - rock that was once live with bacteria and organisms but has since been dried out, can also be a source of nitrate and phosphate, so always check.

Biological media

For about two decades, biological filter media of all types were blamed for being “nitrate factories.” It is true to an extent, they are, but we designed them to be that way, and nitrate is better than ammonia, for any organism. Tanks with lots of biological media can produce lots of nitrate, but only in relation to the amount of ammonia being produced in the first place, so lots of fish and lots of feeding produce lots of ammonia, and subsequent nitrate. 

To combat this, users of the Berlin System employed the use of large Protein Skimmers, and removed biological media altogether, preferring the use of live rock in the main tank, and just a skimmer and chemical media in the sump below. Part of a protein molecule is ammonia, so remove the protein before bacteria have a chance to break it down, and you remove what would be the resulting nitrate. Lots of live rock was used, at the rate of one kilo per ten litres of tank water, so the rockscape became one living biological filter, and the Berlin System went on to form the basis for many reef systems still used and thriving today.

But biological media is back, mainly because we don’t use live rock any more, and because for the ceramic rock we now use, we use a lot less of it. Reefkeepers concerned about the lack of available surface area to break down ammonia (and some don’t even have a sand bed in the main tank,) use dedicated biological media, usually ceramic, to act as a safety barrier for any ammonia which may become present. It’s still the same equation as before - the amount of nitrate that any system produces will be in direct relation to the amount of ammonia going in, so some tanks with high biological load and biological media may develop high nitrate levels, and those with few fish, and very sparse feeding may not develop a problem. The good news is that there are now many ways to tackle rising nitrate levels, most of which utilise good old mother nature in some form.

Nitrate control

The first and easiest way to control nitrate is to change the water. As long as the water you replace has a lower level of nitrate than the water you are removing, nitrates will come down. Regular water changes can help problem reef tanks in all sorts of other ways too, like lowering phosphates, replenishing trace elements, and helping to buffer KH, Calcium and Magnesium. 

Next is to control stocking and feeding. A heavily stocked and heavily fed tank will produce more nitrate, so by removing some large fish and feeding less, you may not notice a change on your nitrate test straight away, but if you were to plot the build up of nitrate on a graph, it should help to slow the increase. 

Mechanical filters like filter socks and Roller filters can physically trap organic waste and remove it from the system, preventing it from being broken down by bacteria. And powerful protein skimming can also result in less nitrate production, again by pre-filtering before it goes through the biological filtration process. 

But even after nitrate has been produced, it can still be tackled effectively by popular modern methods. Macroalgae (tropical seaweeds,) in the form of Caulerpa, in an algae refugium, or Chaetomorpha, in a refugium or an algae reactor will happily utilise nitrate and phosphate as fertiliser, and lock it up into new vegetative growth. Harvest the new growth and pass it on or throw it away, and you’re removed those nutrients from the system. Some also believe that Macroalgae has allelopathic properties whereby it can help to fight the growth of other nuisance algaes, using natural, chemical means. Algae scrubbers aren’t as popular in the UK as algae reactors, but they can also be utilised by pumping system water over an exposed, brightly lit surface. Algae grows in the light and nutrients and is then harvested and removed.


Denitrification is the process of further converting nitrate using other bacteria, into Nitrogen gas. Heterotrophic bacteria, when given an additional carbon based food source, will also happily convert nitrate and reduce it entirely, and nitrate “removers” are available from many companies and brands, and vary from one bottle of bacteria, to one bottle of bacteria and one bottle of food, to just food, to be dosed daily and encourage a population of nitrate converting bacteria somewhere in the system. All of these are best when combined with Protein skimming and biological media.

Bio pellets also consume nitrate and phosphate by providing a large carbon based food resource in pellet form, for bacterial flocs to develop on. Pellets need a dedicated pellet reactor, a pump to power the reactor and violently agitate them, and a protein skimmer to remove old, dead biofloc and waste products from the process. Probiotics, pellets and algae reactors should all be used in isolation from each other, as they will all compete for the nitrogenous food source and risk being starved. You would be wasting money too. 
All can be very effective, and are capable of reducing the nitrate levels to zero on a standard test kit. Many successful reefkeepers will have tried all three methods at some point, and low levels of nitrate can mean less nuisance algae, and better coral colouration. Basic nitrate removal resins are also available, for a quick, chemical removal fix.

Too little nitrate

How low is too low though? Nitrate levels in the Ocean are around 0.42mg/l, and that’s the level that a captive reef system should aim to emulate. Use enough probiotic bacteria and zeolite, to chemically remove ammonia before it is converted, and it is possible to go below that level, but it is no real benefit and corals actually need some nitrate. Clams especially. 

Levels up to 10mg/l are fine for the average reef aquarium, 20mg/l or more should be avoided, and anything above 10mg/l would be considered high nitrate levels in modern terms, although levels of over 100mg/l are common in fish-only marine systems with lots of fish, lots of biological media, and no control methods other than partial water changes. 

Probiotic bacteria seem to take up nitrate in preference to phosphate, so many reefkeepers report zero nitrate, but still a level of phosphate. Nitrate and phosphate levels correlate in something called the Redfield Ratio, and if they go out of kilter, algae can still occur, even with nitrate at zero.